Note 3 - Revenue - Revenues Disaggregated by Product Technology (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues	$ 7,851	$ 7,988	$ 19,850	$ 20,736
Air Pollution Control [Member]
Revenues	3,224	3,711	9,491	10,692
Air Pollution Control [Member] | Technology Solutions [Member]
Revenues	2,554	1,910	6,837	7,449
Air Pollution Control [Member] | Spare Parts [Member]
Revenues	424	1,339	1,428	1,938
Air Pollution Control [Member] | Ancillary Revenue [Member]
Revenues	246	462	1,226	1,305
FUEL CHEM [Member]
Revenues	4,627	4,277	10,359	10,044
FUEL CHEM [Member] | Technology Solutions [Member]
Revenues	$ 4,627	$ 4,277	$ 10,359	$ 10,044